Increased morning incidence of myocardial infarction in the ISAM Study: absence with prior beta-adrenergic blockade. ISAM Study Group.

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Abstract
The time of acute myocardial infarction was determined in all 1,741 patients of the ISAM (Intravenous Streptokinase in Acute Myocardial Infarction) Study, based on onset of clinical symptoms and evaluation of plasma CK-MB enzyme time-activity curves. The incidence of myocardial infarction was markedly increased between 6:00 AM and 12:00 noon compared with other times of day (p less than 0.001). Myocardial infarction occurred 3.8 times more frequently between 8:00 and 9:00 AM (hour of maximum incidence) than between 12:00 midnight and 1:00 AM (hour of minimum incidence). Time of myocardial infarction based on clinical and enzymatic methods correlated well (r = 0.95). Patients with higher or lower left ventricular ejection fraction, higher or lower degree of wall motion abnormalities and residual stenosis of the coronary arteries, and one-, two-, or three-vessel disease exhibited a similar circadian pattern, suggesting that the morning is a risk period for patients with mild as well as severe coronary arter...